The present invention relates generally to an ampltude detection circuit, and more particularly, to an amplitude detector for use in conjunction with an engine imbalance testing system.
Prior to installation in an automobile, an engine is tested for rotational imbalance. Weights are then added to or subtracted from the engine to counteractingly reduce the imbalance to an acceptable level.
Rotational imbalance is sensed and measured by a test system, including an engine mount, or test stand, and a transducer. The engine is placed in the test stand and operated at the test speed (RPM). The transducer is coupled to the engine and provides an electrical signal. A component of that signal represents engine imbalance.
The imbalance component is a sinusoidal signal, having a substantially constant amplitude and a frequency dependent upon RPM. More particularly, the period of the imbalance component is identical to the time between successive top dead center points of the engine.
To determine the imbalance and the necessary corrective action, only the amplitude of the imbalance component must be measured. The transducer signal must, therefore, be demodulated to segregate the imbalance component.
Demodulation is particularly difficult due to variation in RPM and resulting variation in the frequency of the imbalance component. Presently available demodulation circuits also add a DC bias to the output signal, such that the output amplitude is dependent upon the amplitude of the demodulating signal. Variations in the demodulating signal will thus reduce the accuracy of measurement.